Project Description

A geotechnical investigation was conducted alongside the proposed development comprising of two residential towers; a 38 storey south tower and a 51 storey north tower.  The development includes 5 levels of underground parking.

In September 2006, a portion of an east municipal laneway settled under the weight of a waste disposal truck.  The dimensions of the subsurface cavity or sinkhole varied from 7 m in the north-south direction to 3 m in the east-west direction, with the depth approaching 3 m.  The cavity was backfilled by the City of Toronto with unshrinkable concrete fill and the laneway surface paved with asphaltic concrete.

From the ground surface, surficial asphalt or fill materials were encountered to varying depths.  Underlying the surficial materials, compact to very dense sandy silt till / silty sand till was encountered to depths of 8 m.  Discontinuous very stiff to hard silty clay / silty clay till and clayey silt till were encountered above and below the sandy silt till.  A continuous layered silty fine sand aquifer was encountered below the upper discontinuous deposits of glacial till.  The silty sand deposit was very dense with N values in excess of 100 blows for 300 mm penetration extended to depths of 17 m.  Water content determinations indicated that the sand was saturated.  A continuous very dense grey silt stratum with lenses and layers of silty sand and silt was encountered below the silty sand.  The silt stratum extended down to 24 m.  A hard silty clay was encountered underlying the silt stratum down to 30 m.  A hard silty clay till extending to 36 m was encountered underlying the hard silty clay.  Very dense silty sand / sandy silt / sandy silt till, typical of deltaic deposits from the Scarborough Formation were encountered to 44 m below the silty clay till.  Over the site, the average local groundwater table was at a depth of approximately 8 m in April 2003.

A geophysical survey with Ground Penetrating Radar (GPR) of the municipal laneway was conducted by Sarafinchin in November 2006.

A void filling and compaction grouting program was carried out to fill subsurface cavities and to  improve the relative soil density of the very loose to loose fine sands encountered at depths varying from 6 to 12 m at the borehole locations in the east municipal laneway.

No foundation soil disturbance to the two high rise buildings was observed. A technical review by the City of Toronto and Golder Associates Ltd. was in agreement with this soil density improvement approach to mitigate the risks of further deep seated ground subsidence or sinkholes in the east municipal laneway. The soil density improvement  work was successfully completed with final City approval.